Method for anodizing silicon substrates for surface type acceleration sensors

ABSTRACT

An epitaxial growth layer, an oxide film, and a passivation film are formed on a silicon substrate. Except for an opening formed on a part of the passivation film, the upper surface of the passivation film is covered with a metal protective film made of tungsten (W). With the silicon substrate immersed in a high-concentration hydrofluoric aqueous solution, anodization is performed with the silicon substrate as an anode and the metal protective film as a counter electrode.

BACKGROUND OF THE INVENTION

The present invention relates to a method for anodizing siliconsubstrates and a method for manufacturing a surface-type accelerationsensor using the anodizing method.

Anodization of silicon substrates has conventionally been practiced inmicro machining of silicon. FIG. 18 shows a conventional method ofanodization. A silicon substrate 29 and a counter electrode 31 areimmersed in an aqueous HF solution (aqueous hydrofluoric solution). Thesilicon substrate is an anode and the counter electrode 31 is made, forexample, of Pt (platinum). An electric field is applied to the siliconsubstrate 29 and the counter electrode 31. The silicon substrate 29includes a first portion, which will be porous, and a second portion,which excludes the first portion. The surface of the second portion ofthe silicon substrate 29 is covered with a resin film (protection film)30 such as a photoresist for protection from the HF solution 27. Duringanodization, the first portion of the silicon substrate 29 becomes aporous silicon layer 25. In a later step, the layer 25 is removed byalkali etching to form a cavity in the silicon substrate 29.

However, since the resin film 30 does not closely contact the siliconsubstrate 29, the HF solution 27 may enter the space between the film 30and the silicon substrate 29. This erodes the second portion of thesilicon substrate 29.

To solve the above problem, a ceramic film, which has HF resistance, maybe used instead of the resin film 30. However, the ceramic film is hardto form and is made by a different manufacturing process than an ICmanufacturing process. This makes the ceramic film unsuitable forforming elements such as an acceleration sensor, the manufacturingprocess of which is close to the IC manufacturing process.

An objective of the present invention is to provide a method ofanodization in which only a required portion of a silicon substrate ismade porous.

Another objective of the present invention is to provide a method formanufacturing an improved surface-type accelerator sensor using theanodization method.

SUMMARY OF THE INVENTION

To achieve the above objectives, in the anodization of a siliconsubstrate of the present invention, the silicon substrate includes afirst portion, which is made porous, and a second portion, whichexcludes the first portion. A metal protective film having HF resistanceis formed on the surface of the second portion of the silicon substrate.During the formation of the metal protective film, a metal silicidehaving HF resistance is formed between the metal protective film and thesilicon substrate. The silicon substrate covered with the metalprotective film is immersed in the HF solution, a positive terminal of adirect current power source is connected to the silicon substrate, whichserves as an anode, and a voltage is applied to the silicon substrate.

In the present invention, the metal protective film and the metalsilicide are made of HF resistant metals, which include W (tungsten) andMo (molybdenum). When the silicon substrate is immersed in and eroded bythe HF solution during anodization, the portion covered with the metalprotective film and the metal silicide are effectively protected. As aresult, the surface of the silicon substrate covered by the metalprotective film is not corroded by the HF solution.

In a method of the present invention for manufacturing a surface-typeacceleration sensor, a first p-type silicon layer is formed on apredetermined area of a surface of a p-type single crystal siliconsubstrate by adding impurities. An epitaxial growth layer, which is madeof n-type single crystal silicon, is formed on the upper surface of thep-type single crystal silicon substrate, such that the first p-typesilicon layer is covered by the epitaxial growth layer. A second p-typesilicon layer for forming an opening portion is formed in the epitaxialgrowth layer by adding impurities. A deformation gage, which is made ofp-type silicon, is formed on the upper surface of the epitaxial growthlayer. A wiring pattern, which is connected to the deformation gage, isthen formed. A passivation film is formed to cover the wiring patternwith the second p-type silicon layer exposed. A metal protective filmhaving HF resistance is formed on the surface of the passivation filmand the epitaxial growth layer but not on the second p-type siliconlayer. A metal silicide having HF resistance is formed between the metalprotective film and the epitaxial growth layer. Anodization is performedto convert the first and second p-type silicon layers into poroussilicon layers. The porous silicon layers are removed by alkali etchingto form a beam, which is made of the epitaxial growth layer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view showing a surface-typeacceleration sensor according to a first embodiment of the presentinvention;

FIG. 2 is a diagrammatic plan view showing the acceleration sensor ofFIG. 1;

FIG. 3 is a diagrammatic cross-sectional view showing the accelerationsensor when mounted on a substrate;

FIG. 4 shows an equivalent circuit diagram of the acceleration sensor;

FIG. 5(a) is a diagrammatic cross-sectional view showing theacceleration sensor during a first manufacturing step;

FIG. 5(b) is a diagrammatic plan view of the sensor of FIG. 5(a);

FIG. 6(a) is a diagrammatic cross-sectional view of the sensor in asecond manufacturing step, following the step of FIG. 5(a);

FIG. 6(b) is a diagrammatic plan view of the sensor of FIG. 6(a);

FIG. 7(a) is a diagrammatic cross-sectional view of the sensor in athird manufacturing step, following the step of FIG. 6(a);

FIG. 7(b) is a diagrammatic plan view of the sensor of FIG. 7(a);

FIG. 8(a) is a diagrammatic cross-sectional view of the sensor in afourth manufacturing step, following the step of FIG. 7(a);

FIG. 8(b) is a diagrammatic plan view of the sensor of FIG. 8(a);

FIG. 9(a) is a diagrammatic cross-sectional view of the sensor taken online 9 a— 9 a in a fifth manufacturing step following the step of FIG.8(a);

FIG. 9(b) is a diagrammatic plan view of the sensor of FIG. 9(a);

FIG. 10(a) is a diagrammatic cross-sectional view of the sensor taken online 10 a— 10 a in a sixth manufacturing step, following the step ofFIG. 9(a);

FIG. 10(b) is a diagrammatic plan view of the sensor of FIG. 10(a);

FIG. 11 is a diagrammatic cross-sectional view of the sensor in aseventh manufacturing step, following the step of FIG. 10(a);

FIG. 12 is a diagrammatic cross sectional view of the sensor in a methodof anodization;

FIG. 13 is a diagrammatic cross-sectional view showing the accelerationsensor after the anodization step;

FIG. 14 is a diagrammatic perspective view showing a plurality ofacceleration sensors formed on a wafer according to a second embodimentof the present invention;

FIG. 15 is a diagrammatic perspective view showing the wafer in amanufacturing step following the step of FIG. 14;

FIG. 16 is a diagrammatic cross-sectional view showing an anodizationstep of the second embodiment;

FIG. 17 is a diagrammatic cross-sectional view showing a surface-typeacceleration sensor of the second embodiment; and

FIG. 18 is a diagrammatic cross-sectional view showing a prior artmethod of anodization.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

A first embodiment of the present invention will now be described withreference to FIGS. 1-13. As shown in FIGS. 1 and 2, a surface-typeacceleration sensor 1 of the first embodiment includes a p-type singlecrystal silicon substrate (hereinafter referred simply to as siliconsubstrate) 2. The silicon substrate 2 has an orientation flat (110). Thesilicon substrate 2 of the acceleration sensor 1 is formed on a wafer byscribing, which is different from later mentioned manufacturing steps.

A generally square recess 3 is located in the top central surface of thesilicon substrate 2. The recess 3 is formed by removing a porous p-typesilicon layer by alkali etching. A cantilever beam 5, which is supportedby a single side, is located above the recess 3 to move vertically. Thecantilever beam 5 is mainly made of an n-type single crystal siliconepitaxial growth layer 6. Four diffusion-type deformation gages 7, whichare made of p-type silicon by adding impurities, are formed on the topsurface of the proximal end of the cantilever beam 5.

A thin oxide film (SiO₂) 8 which serves as an insulating layer betweenlayers is formed on the epitaxial growth layer 6. A wiring pattern 9 anda bonding pad 10 are formed on the oxide film 8 by a physical filmforming method, such as sputtering or vacuum vapor deposition. Contactholes 11 for connecting layers are formed on the ends of the deformationgages 7. The contact holes 11 are used to electrically connect thewiring patterns 9 to the deformation gages 7 below the wiring patterns9. The wiring patterns 9 are electrically connected to the bonding pads10, which are exposed on the peripheral upper surface of the siliconsubstrate 2. A thin passivation film 12 for insulation at the top layeris formed on the upper surface of the oxide film 8 by a physical filmforming method. Openings are formed on predetermined portions of thepassivation film 12 to expose the bonding pads 10. The epitaxial growthlayer 6, the oxide film 8, and the passivation film 12 form a generallyU-shaped opening 4.

The preferred dimensions of each part of the acceleration sensor 1 areas follows. The thickness t of the silicon substrate 2 (including theepitaxial growth layer 6) is about 500 μm, and the width w is 500μm-1000 μm. The thickness of the cantilever beam 5 is about 10 μm, thewidth is about 200 μm, and the length is about 300 μm. A clearancebetween the lower surface of the cantilever beam 5 and the bottomsurface of the recess 3 is about 10 μm. The thickness of the oxide film8 is about 0.5 μm, and the thickness of the passivation film 12 is alsoabout 0.5 μm.

As shown in FIG. 3, the acceleration sensor 1 is installed on anothersubstrate (motherboard) 13. A die bond material 14 is applied to theentire lower surface of the silicon substrate 2, and the siliconsubstrate 2 is attached to the motherboard 13 with the die bond material14. The bonding pads 10 of the silicon substrate 2 are electricallyconnected to a bonding pad 15 of the motherboard 13 with a wire 16 viawire bonding. Further, the motherboard 13 includes a signal processingcircuit (not shown), which calculates acceleration based on the outputvoltage from the acceleration sensor 1.

The deformation gages (that is, diffusion deformation resistors R1-R4)are connected as shown in FIG. 4. Of the bridge-connected four resistorsR1-R4, a first node between the resistor R1 and the resistor R4 isconnected to a bonding pad 10 for supplying voltage Vcc from a powersource. A second node between the resistor R2 and R3 is connected to abonding pad 10 which may be connected to ground. Nodes between theresistors R3 and R4 and between the resistors R1 and R2 are respectivelyconnected to two bonding pads 10 for the output of the sensor 1. FIG. 2shows a schematic layout of the resistors R1-R4 in the cantilever beam5. The resistors R1-R4 are substantially aligned in the widthwisedirection of the cantilever beam 5. The longitudinal direction of theresistors R1, R3 is parallel to the direction in which the cantileverbeam 5 extends. The longitudinal direction of the resistors R2, R4 isperpendicular to the direction in which the cantilever beam 5 extends.Accordingly, the resistors R1, R3 extend in the direction of [110], andthe resistors R2, R4 extend in the direction of [/1, /1, 0].

When acceleration is applied to the acceleration sensor 1 as shown bythe arrow A1 of FIG. 1, the proximal portion of the cantilever beam 5curves and the entire cantilever moves downward. This deforms thedeformation gages 7 (resistors R1-R4), which either increases ordecreases the resistance of each deformation gage 7 as caused by thePiezo resistance effect of silicon. Acceleration is detected bydetecting the changes of resistance of each deformation gage 7.

A method for manufacturing the acceleration sensor 1 of the presentembodiment will now be described with reference to FIGS. 5-13. In themanufacturing method, the silicon substrate 2 is formed on a wafer (notshown for simplification of illustration).

First, as shown in FIGS. 5(a), 5(b), the p-type single crystal siliconsubstrate 2, which is a generally rectangular parallelepiped and has anorientation flat (110), is prepared. A mask (not shown) is formed on thesurface of the silicon substrate 2. Subsequently, boron is introducedinto the silicon substrate 2 by ion implantation, and the boron isdiffused by heat. As a result, as shown in FIGS. 6(a) and 6(b), a p-typesilicon layer 21 is formed substantially in the center of the siliconsubstrate 2.

As shown in FIGS. 7(a) and 7(b), the epitaxial growth layer 6, which ismade of n-type single crystal silicon, is formed by vapor phase epitaxyon the upper surface of the silicon substrate 2 that includes the p-typesilicon layer 21. As a result, p-type silicon layer 21 is embedded inthe epitaxial growth layer 6. Then, a mask (not shown) is formed on thesurface of the epitaxial growth layer 6, and a generally U-shapedopening is formed on a predetermined area of the mask by photo etching.

Then, boron is introduced into the silicon substrate 2 by, for example,ion implanting, and the boron is diffused by heat to form the p-typesilicon layer 22 for the U-shaped opening as shown in FIG. 8. The p-typesilicon layer 22 is flush with the surface of the epitaxial growth layer6 and contacts the p-type silicon layer 21

Then, the mask (not shown) is arranged on the upper surface of theepitaxial growth layer 6. Boron is introduced into the silicon substrate2 by ion implantation and the boron is diffused by heat, which formsfour deformation gages 7 at a part that will later become the uppersurface of the proximal end of the cantilever 5. Next, the siliconsubstrate 2 is heated in oxygen or air, which forms the oxide film 8 onthe upper surface of the silicon substrate 2. Subsequently, photoetching is performed on the oxide film 8, which forms the contact hole11 at a predetermined portion of the oxide film 8 as shown in FIG. 9.

Then, photolithography is performed on the silicon substrate 2 aftersputtering or vacuum vapor deposition using Al (aluminum) is carriedout. This forms the wiring pattern 9 and the bonding pads 10.Subsequently, as shown in FIG. 10, the passivation film 12 is formed onthe upper surface of the silicon substrate 2 to cover the wiringpatterns 9 by depositing SiN or Si₃N₄ by CVD (chemical vapordeposition). In the formation of the passivation film 12, openings 12 afor exposing the bonding pads 10 and a generally U-shaped opening 12 bare formed in the passivation film 12. Then, part of the oxide film 8that is over the p-type silicon layer 22 is removed, which exposes theupper surface of the p-type silicon layer 22.

Subsequently, the entire upper surface of the passivation film 12 iscovered by the metal protective film 23, which is made of W (tungsten),by a physical film forming method, such as sputtering or vacuum vapordeposition. During the film formation, below the opening 12 b (i.e. atthe opening of the oxide film 8), W silicide is formed on the interfacebetween the metal protective film 23 and the epitaxial growth layer 6. W(tungsten), which forms the metal protective film 23, and W silicide areHF-resistant. In the next step, as shown in FIG. 11, the generallyU-shaped opening 24 is formed above the p-type silicon layer 22 byphotolithography.

As shown in FIG. 12, the silicon substrate 2 is immersed in ahigh-concentration HF aqueous solution as a solution containing HF. Apositive terminal of a DC electrode V is connected to the siliconsubstrate 2, and the negative terminal is connected to the metalprotective film 23. The silicon substrate 2 is an anode, and the metalprotective film 23 is a counter electrode. Voltage is applied betweenthe silicon substrate 2 and the metal protective film 23. At this time,the applied voltage does not exceed 0.6V and optimum voltage is applied.In this way, only the p-type silicon layers 21, 22 are selectivelyturned porous by anodization, which converts only the designated areasinto the porous silicon layer 25. The portion that is covered by themetal protective film 23 is not corroded by the HF aqueous solution.Since HF-resistant W silicide is formed on the interface between themetal film 23 and the epitaxial growth layer 6 in the opening 12 b, theHF aqueous solution does not penetrate the interface and corrode theinside.

Subsequently, the porous silicon layer 25 is subjected to anisotropicetching by means of alkali etching using THAM (tetramethyl ammoniumhydroxide). The p-type silicon layers 21, 22 that have undergone theanodization and have become porous are readily soluble in alkali. As aresult, the cavity 26 is easily formed by removing the porous siliconlayer 25 (See FIG. 13). Finally, the metal protective film 23, which isnow no longer necessary, is removed by etching such as plasma etching,which completes the acceleration sensor 1 as shown in FIG. 1.

In the acceleration sensor 1 of the present invention, the cantileverbeam 5 is mainly made of n-type single crystal silicon epitaxial growthlayer 6. Therefore, the deformation gage 7, which is made of p-typesilicon having a relatively large gage factor, can be formed on theupper surface of the epitaxial growth layer 6. This makes theacceleration sensor 1 more sensitive than the conventional accelerationsensors having a deformation gage made of n-type silicon.

The acceleration sensor 1 is a so-called surface sensor, which ismanufactured without performing anisotropic etching on the lower surfaceof the silicon substrate 2. Therefore, the problem in conventionalbulk-type acceleration compressors (that is, the width of a chip width Wis increased by forming etching holes along the flat (111)) is solved.The size of the acceleration sensor 1 is reduced without degrading thepredetermined detection sensitivity. Further, the cantilever beam 5 ofthe surface-type acceleration sensor 1 is not exposed from the bottomsurface of the silicon substrate 2 and does not contact the die bondmaterial 14 and motherboard 13. Accordingly, a seat for the accelerationsensor 1 is not required, which facilitates installation of theacceleration sensor 1.

The manufacturing method of the present embodiment has the followingadvantages.

(1) In the prior art anodization of FIG. 18, the area of the counterelectrode 31 must be equal to the area of the silicon substrate 29 toperform an equal anodization. In contrast, in the anodization of thepresent embodiment, the metal protective film 23 is used as a counterelectrode, which does not necessitate a counter electrode that is madeof a precious metal plate. Therefore, the anodization of the presentembodiment is less expensive than that of the prior art. Also, since themetal protective film is formed on substantially the entire surface ofthe silicon substrate 2, anodization is equally performed.

Further, since the metal protective film 23, which is located close tothe silicon substrate 2, is used as a counter electrode, the resistanceof the HF aqueous solution does not have to be considered. In the priorart anodization, since the silicon substrate is distant from the counterelectrode, it is necessary to control the current or the voltage of theDC power source V during anodization considering the resistance of theHF aqueous solution. In the present embodiment, anodization isfacilitated since the resistance of the HF aqueous solution does nothave to be considered.

(2) During anodization of the present embodiment, the applied voltagedoes not exceed 0.6V. If voltage that exceeds 0.6V is applied duringanodization, leakage current (invalid current), which is not relevant toanodization, flows in the pn junction of the silicon substrate 2 and theepitaxial growth layer 6 through the wiring pattern 9, since the pnjunction forms a diode. In the present embodiment, since voltage that isless than 0.6V is applied and the flow of leakage current is prevented,anodization is efficiently performed.

(3) The metal protective film 23, which is made of W (tungsten), has ahigh fusion point and a coefficient of thermal expansion that is closeto that of the passivation film 12 made of SiN or Si₃N₄ and theepitaxial growth layer 6. Therefore, the metal protective film 23closely contacts and does not peel off of the passivation film 12 andthe epitaxial growth layer 6.

(4) The p-type silicon layers 21, 22 are formed on predetermined areasin advance and are then anodized. Therefore, the anodized portions haveuniform shape and depth in comparison with the prior art anodization, inwhich the surface of the silicon substrate 2 is directly anodized.

(5) It is not so difficult to form the epitaxial growth layer 6 on thep-type silicon layer 21.

(6) The anodization is performed after the passivation step iscompleted, which enables forming the metal protective film 23 when thecavity 26 has not yet been formed. This facilitates the formation of themetal protective film 23.

In other words, since the metal protective film 23 does not get insidethe cavity 26, there is no need to remove the metal protective film 23.Also, since the alkali etching is performed after the passivation step,the wiring patterns 9 and the bonding pads 10 are not contaminated byetchant. Therefore, the manufacturing of the acceleration sensor 1 issimplified, which facilitates the work.

(7) The manufacturing method, in which the porous silicon layer 25 isremoved, is not restricted by the orientation flat of the siliconsubstrate 2.

Also, the manufacturing method of the present invention (including theanodization method using tungsten as the metal protective film 23), issimilar to the method of manufacturing bipolar IC, in which W (tungsten)is used as a material for the gates of the transistors. Accordingly, theacceleration sensor 1 and the bipolar IC can be integrated on the samecircuit, which contributes to reducing the size and increasing the speedof the acceleration sensor 1.

Second Embodiment

A method of manufacturing the acceleration sensor 1 according to asecond embodiment of the present invention will now be described withreference to FIGS. 14-17. The same reference numerals are used to referto the same members as those of the first embodiment, and thedescription for such members is omitted.

The second embodiment further equalizes the potential applied to thesilicon substrate 2 than in the first embodiment. FIG. 14 shows thesilicon substrate 2 before subscribing, and FIG. 15 shows a wafer thatis covered by the metal protective film 23. In FIGS. 14 and 15, theexposed portion and the opening of the p-type silicon 22 are enlarged,and the wiring patterns 9, the bonding pads 10, and the deformationgages 7 are omitted to facilitate illustration.

The steps of the manufacturing method of the present invention are thesame as the steps of the first embodiment up to the step shown in FIG.9. The rest of the steps of the second embodiment will now be described.The wiring patterns 9, the bonding pads 10, and conductive patterns 28are formed on the silicon substrate 2 by photolithography aftersputtering and vacuum vapor deposition using Al (aluminum). As shown inFIG. 14, the conductive patterns 28 are formed in a lattice structurearranged between the portions that will be the sensors on each siliconsubstrate (wafer) 2.

Subsequently, as shown in FIG. 16, the passivation film 12 is formed tocover the wiring patterns 9 on the upper surface of the siliconsubstrate 2 by depositing SiN or Si₃N₄ by, for example, CVD. In thepassivation step, the bonding pads 10, openings 12 a for exposing thebonding pads 10 and the conductive patterns 28, the generally U-shapedopening 12 b, and the openings 12 c are formed on the passivation film12. Then, the oxide film 8 on the upper surface of the p-type siliconlayer 22 is removed to expose the upper surface of the p-type siliconlayer 22.

Subsequently, the entire upper surface of the passivation film 12 iscovered by the metal protective film 23 made of W (tungsten) by aphysical film forming method such as sputtering or vacuum vapordeposition. In the film formation, W silicide is formed on the interfacebetween the metal protective film 23 and the epitaxial growth layer ofthe silicon substrate 2 below the opening 12 b (i.e. the opening of theoxide film 8). Also, the conductive patterns 28 are electricallyconnected to the metal protective film 23 via the openings 12 c.

Then, as shown in FIG. 15, a generally U-shaped opening 24 is formedabove the upper surface of the p-type silicon layer 22.

To perform anodization treatment, the silicon substrate 2 is immersed inan HF aqueous solution, which is a solution including HF as in the firsttreatment. In this state, current is applied to the silicon substrate 2as an anode and the metal protective film 23 as a counter electrode (SeeFIG. 16, HF aqueous solution is not shown). In this embodiment also, theapplied voltage is less than 0.6V.

In the above anodization, only the p-type silicon layers 21, 22 areturned into porous silicon layers 25. During the anodization, theportion that is covered by the metal protective film 23 is preventedfrom being corroded by the HF aqueous solution. Also, since HF-resistantW silicide is formed on the interface between the metal protective film23 and the epitaxial growth layer 6 of the silicon substrate 2 at theopening 12 b, the HF aqueous solution does not penetrate the interfaceand corrode the interior.

Subsequently, alkali etching is performed as in the first embodiment.The cavity 26 is formed by removing the porous silicon layers 25 byanisotropic etching. Finally, the metal protective film 23 is removed byetching such as plasma etching, and each element is scribed, whichresults in the acceleration sensor 1 shown in FIG. 17.

The second embodiment has the following advantages.

(1) In this embodiment, the conductive patterns 28 are arranged like alattice located between each element. The conductive patterns comprisealuminum (resistance rate ρ=2.7 μΩcm), which has a lower resistance ratethan W (tungsten: resistance rate ρ=5.5 μΩcm), which forms the metalprotective film 23. Accordingly, since current flows to the conductivepatterns 28 having a lower resistance rate than the metal protectivefilm 23, distribution of potential to the wafer surface is improved, orrelieved. This equalizes current and achieves equalized anodization.

Further, the metal protective film 23 may be made of Mo (molybdenum)instead of W (tungsten). In this case also, the resistance rate of theconductive patterns 28 is less than that of molybdenum (ρ=5.2 μΩcm),which results in the same effect.

(2) In this embodiment, the conductive patterns 28 are made of the samematerial (aluminum) as the wiring patterns 9 and formed in the samemanufacturing step as the wiring patterns 9 that forms the circuit. Thisfacilitates the formation of the conductive patterns 28.

The present invention can further be varied as follows.

(1) Instead of a substrate having the orientation flat (110), asubstrate having, for example, an orientation flat (111) or (100) may beused. In the first embodiment, the sensitivity of the accelerationsensor 1 is improved by using a substrate having the orientation flat(100).

(2) An alkaline etchant such as KOH, hydrazine, or EPW(ethylenediamine-pyrocatechol-water) may be used in place of THAM.

(3) The wiring patterns 9 and the bonding pads 10 are made of metalsother than aluminum (Al), such as, gold (AU).

(4) An n-type multiple crystal silicon layer or amorphous silicon layermay be used to manufacture the acceleration sensor 1 instead of then-type single crystal silicon epitaxial growth layer.

(5) Instead of the deformation gage 7 in the first embodiment, a filmdeformation gage made of, for example, chromium (Cr) or polycrystallinesilicon may be used.

(6) In the first embodiment, a mass portion may be formed on the lowerside of the distal portion of the cantilever beam 5.

(7) A bipolar IC that functions as a logic circuit may be formed on thesurface of the silicon substrate 2 around the cantilever structure.

(8) In the first embodiment, instead of the W (tungsten) as the metalprotective film, Mo (molybdenum) may be used. This results in the sameeffect. The metal protective film made of molybdenum has a high fusionpoint as tungsten has and has a thermal expansion coefficient close tothose of the passivation film 12 and the epitaxial growth layer 6 of thesilicon substrate 2, which are made of SiN or Si₃N₄. Therefore, themetal protective film is in close and firm contact with the passivationfilm 12 and the epitaxial growth layer 6.

(9) In the first and second embodiments, the metal protective filmserves as the counter electrode during anodization. However, a preciousmetal plate such as of Pt may be used as the counter electrode as in theprior art.

Some technical terms in the specification are defined as follows.

Cantilever structure means a portion that shifts when acceleration isapplied. For example, the cantilever structure may have a mass portionsupported by one or more beams. Also, the cantilever structure may beonly one cantilever beam not including the mass portion.

Anodization is a comprehensive quality change treatment, in whichcurrent is applied to a substrate as an anode in an electrolyte, and aporous layer is formed on the substrate.

What is claimed is:
 1. A method for manufacturing a surface-typeacceleration sensor comprising the steps of: forming a first p-typesilicon layer on a predetermined area of a surface of a p-type singlecrystal silicon substrate by adding impurities; embedding the firstp-type silicon layer below an epitaxial growth layer by forming theepitaxial growth layer, which is made of n-type single crystal silicon,on an upper surface of the first p-type single crystal siliconsubstrate; forming a second p-type silicon layer for forming an openingin the epitaxial growth layer by adding impurities; forming adeformation gage, which is made of p-type silicon, on an upper surfaceof the epitaxial growth layer; forming a wiring pattern connected to thedeformation gage; forming a passivation film over the wiring patternwhen the second p-type silicon layer is exposed; forming an HF-resistantmetal protective film on a surface of the passivation film and theepitaxial growth layer, but not on the second p-type silicon layer;forming an HF-resistant metal silicide film on the interface between themetal protective film and the epitaxial growth layer when the metalprotective film is formed; performing anodization to convert the firstand second p-type silicon layers into a porous silicon layer; andremoving the porous silicon layer by alkali etching to form a beam thatis made of the epitaxial growth layer.
 2. The method for manufacturingthe acceleration sensor according to claim 1, wherein the step ofanodization includes a step of applying voltage to the silicon substratethat is covered with the metal protective film when the siliconsubstrate is immersed in a solution containing HF, and when a positiveterminal of a DC power source is connected to the silicon substrate. 3.The method for manufacturing the acceleration sensor according to claim2, wherein the step of anodization further includes a step of applyingthe voltage to the metal protective film together with the siliconsubstrate when a negative terminal of the DC power source is connectedto the metal protective film.
 4. The method for manufacturing theacceleration sensor according to claim 3, further including a step offorming a conductive pattern on a second portion of the siliconsubstrate excluding a first portion before the formation of the metalprotective film.
 5. The method for manufacturing the acceleration sensoraccording to claim 4, wherein the conductive pattern has a resistancerate lower than that of the metal protective film.
 6. The method formanufacturing the acceleration sensor according to claim 2, wherein thevoltage applied to the silicon substrate is less than about 0.6V.
 7. Themethod for manufacturing the acceleration sensor according to claim 1,wherein the anodization step further includes applying the voltage to acounter electrode, which is located in the HF containing solution and isspaced from the silicon substrate, when a negative terminal of the DCpower source is connected to the counter electrode.
 8. The method formanufacturing the acceleration sensor according to claim 1, furtherincluding a step of removing the metal protective film.
 9. A method foranodizing a silicon substrate having a first portion to be made porousand a second portion that excludes the first portion, the methodcomprising the steps of: forming an HF-resistant metal protective filmon a surface of the second portion of the silicon substrate; forming anHF-resistant metal silicide film on the interface between the metalprotective film and the silicon substrate when the metal protective filmis formed; immersing the substrate in a solution containing HF; andapplying a voltage to the silicon substrate when the silicon substrateis immersed in the solution containing HF, and when a positive terminalof a DC power source is connected to the silicon substrate.
 10. Themethod of anodization according to claim 9, wherein the voltage applyingstep includes connecting a negative terminal of the DC power source tothe metal protective film.
 11. The method of anodization according toclaim 10 further including a step of forming a conductive pattern on thesurface of the second portion of the silicon substrate before theformation of the metal protective film.
 12. The method of anodizationaccording to claim 11, wherein the conductive pattern has a resistancerate lower than that of the metal protective film.
 13. The method ofanodization according to claim 9, wherein the voltage applying stepincludes applying the voltage to a counter electrode when a negativeterminal of the DC power source is connected to the counter electrode,which is located in the solution containing HF and is spaced from thesilicon substrate.
 14. The method of anodization according to claim 9,wherein the voltage applied is less than about 0.6V.